My boyfriend asked his friend’s sister to buy him food after doing her a favor. Am I overthinking this?

The other day I saw a conversation on Instagram between my boyfriend and his friend's older sister. She asked him for a favor (she wanted to drop off cash to his brother). After helping her, he told her she should treat him to food because he did her a favor.

I jokingly asked, "So is this like a date?" He laughed it off and said she's over 30, his friend's sister, and "like a sister" to him. He also said I could come with them, but he knows I'm not really the type to tag along.
What I'm stuck on is why he asked her to buy him food in the first place. To me, asking someone to treat you to a meal can feel a bit date-like, but maybe I'm reading too much into it.
Does this sound like normal joking between friends, or would this make you suspicious? I'm looking for outside perspectives.